This invention relates to a method for testing and treating fuel oils such as diesel fuel which needs to be stored or has been stored for long periods of time and in particular in fuel storage situations required for emergency stand-by motor generators.
Current diesel fuels generally require more refining, i.e. catalytic cracking, than heavier oils. The conversion of many standby emergency power facilities to the use of diesel fuel has therefore produced several new problems relating to the inherent instability of such fuels when stored for long periods of time.
Distillate fuels, including diesel fuels are prone to deteriorate during prolonged storage by forming polymerizates which agglomerate into what is referred to as sludge which can clog fuel lines, filters, and fuel injectors preventing, for example, the reliable operation of a diesel engine. In addition, water in the fuel and in the form of condensates in a partially filled distillate fuel storage tank will, by itself or in combination with fuel components, attack the metal of the tank forming rust which also promotes the polymerization of components in the fuel.
In addition, new regulations promulgated by the Environmental Protection Agency have recognized the problem of rusting tanks and now require measures to prevent contamination of ground water which can occur from fuel leaking underground from tanks that have been perforated by rust.
Likewise, sludge formation can be accelerated by the growth of bacteria in the fuel.
Therefore, a wide variety of polymerization inhibitors have been employed. Such inhibitors were generally capable of retarding the formation of sludge and newer compositions, such as described in U.S. patent application, Ser. No. 621,073, filed June 15, 1984, by George Holcom Kitchen, III entitled "Fuel Additive", have shown the additional capability of dispersing agglomerates, polymerizates and sludge once formed. It is known that the deterioration of fuel oils involves polymerization reactions resulting in the agglomeration of macroscopic polymerizates into sludge. Although this reaction may be effected by the presence of oxygen, additives containing antioxidants, such as hindered phenols or diamines of the types used in gasolines as gum inhibitors, are not totally effective for the purpose of preventing the polymerization mechanisms. The additive materials, in addition to the foregoing, should also have rust-preventive properties in order to keep the fuel tanks clean and dry, and to reduce or eliminate rust formation in the tanks.
The additive materials should also inhibit the propagation of bacteria. The kinds of bacteria that grow in stored fuels thrive on nitrogen, sulfur, and phosphorus, as well as iron, generally in the form of its oxides. Of course, the elimination of materials in the fuel tank that contain nitrogen, sulfur or phosphorous is difficult to achieve in any practical situation.
For all of the foregoing reasons, various additive materials are utilized to prolong the storage life of distillate fuels.
Even the best of these prior attempts to stabilize distillate fuels against substantial deterioration during long-term storage were only effective for limited periods of time. Prolonged storage situations still requires routine monitoring and corrective action to reduce agglomeration, rust and the like in order to insure proper engine performance from the stored fuel when its use is required in emergency situations.
In order to determine the continued efficiency of the stored fuel, many different testing procedures were devised to help the user determine whether fuel stored for prolonged periods of time will be capable of performing effectively, when required.
A widely used test has been a variation of the colorstability test in Federal Specification VV-K-211 Kerosene. Generally, this test specified heating a sample of the fuel and observing a color change which was relatable, in some degree to the current condition of the fuel and its ability to withstand further storage without replacement or the addition of further additives. In addition to observing the color change, the amount of filterable sludge and sediment also had to be measured. Another test procedure used was a prolonged version of the Gulf Oil Company's Fuel Corrosion of Steel Test. This test, developed by Bell Laboratories, has been correlated against fuels actually stored in a stand-by power fuel tank. These tests were run at 210.degree. F. until an observable amount of sludge has formed. This test is essentially an accelerated heat-stability test and is typically run in the absence of water. A second test was run at 120.degree. F. over water in the presence of 1020 steel strip. This portion of the test is concluded only after 12 weeks have lapsed or when an observable quantity of rust and sludge has been deposited.
The accelerated heat-stability portion of the foregoing test was comparatively quick and useful for screening out poor additive performance in the selection of additives for use and to a limited extent, for routinely monitoring the condition of actual stored fuel. But, because water is not present in this test, it was not capable of differentiating between those additives that are either not effective rust inhibitors, or not suitable for protecting the fuels when stored in contact with water and steel, from those that are effective under storage conditions where water and steel (and therefore rust) are present. It is precisely these conditions that are the most important in long-term storage since stand-by fuels are frequently in contact with metal and condensate water, and the presence of particulate rust may be often as severe a problem as sludge formation. Even the 12 week stability-and-rust test, which was designed to evaluate these effects did not provide a timely method for monitoring storage conditions and problems.
Because of the importance of stabilizing the fuels for extended periods of time, i.e. up to 20 years, with the fuels in contact with metal and water, it is essential that the performance of the additives also be evaluated properly for use before and during storage.
In another previously used test, artificially aged fuel is filtered and the residues measured by various means. A major problem associated with these tests was the presence of various sized polymers which caused errors in the amount of material retained on the filter pads as well as inordinately lengthening the time for filtration. The new refining technology coupled with the new crude sources has tended to aggravate this problem. Some experimenters have used finer filter paper, others have switched to membrane filters. Both of these changes recognized the presence of the described problem but do not solve it with respect to test speed. Other experimenters attempting to solve the foregoing problems have varied the method of introducing oxygen in the aging process, concentrating on oxidation as the principal mechanism of polymer generation. This also has met with only limited success.
As previously described, the additives previously employed have provided some measure of protection for stored fuel with respect to some of the major properties required, for the short-term.
For very long term storage however, it is essential that the inhibitor employed be capable of being employed both initially and during routine maintenance, if required, to depolymerize and disperse the sludge that is inevitably formed, such as described in U.S. Ser. No. 621,073 which is incorporated herein by reference. More timely results from test procedures are still required using such additives so that the test can be conducted at the site of the stored fuel and the necessary corrective action taken at that time.
It is also important that attempts to eliminate the problem of injector clogging at low temperature by the build-up of hydrocarbon waxes in the fuel by the use of additives containing solvents for these waxes does not compound injector scoring problems by reducing or eliminating the lubricity of the fuel. Over-zealous use of such previously used solvent-containing additives can, and often does produce service interruption due to the mechanical demands of the diesel engine, which can be pressed into prolonged service during emergencies.
Therefore, the combination of new refining technology and the use of new crude sources and the kinds of additives now available have produced a need for an improvement of the current fuel testing procedures.
It is therefore an objective of the present invention to provide a test procedure for stored distillate fuels which can be used at the site of the stored fuel at the time of routine maintenance which will give a timely indication of the condition of the fuel for its suitability for subsequent use and further storage thereby providing an opportunity to employ timely corrective action if required.